Passive infra-red (PIR) devices using the pyroelectric effect and operating in the wavelength region 2 xcexcm to 14 xcexcm are widely used to automatically detect events, such as fires and intruders, where their sensitivity to changes in temperature provides discrimination not available to visible detectors. Examples of this are disclosed in European Patent Application EP 0853237 A1. The sensitivity of a PIR device to changes in temperature rather than absolute temperature makes it especially suitable for detecting movement or the onset of a fire. Arrays of passive infra-red detectors in an imaging system can provide spatial information about the location of events within their field, but their use to recognise events in detail, such as the appearance of an intruder, is limited because passive infra-red imagers of good resolution are very expensive. In the case of pyroelectric detector arrays arrangements must be made to chop or modulate the radiation incident on the detectors to provide a convnetional image of the scene. However if the infra-red radiation is not chopped only changes in temperature or movement are detected, so that the output from the array, which is not a recognisable image in the normal sense, contains only information about any portions of the scene which are changing.
Video cameras are widely used for surveillance, but suffer from the disadvantage that the identification of significant events may require the intervention of a human operator. Also, when the camera covers a wide field, the resolution of the camera may be insufficient to provide for recognition of an intruder or a car number plate. Such cameras require the scene to be continuously illuminated, either by daylight or by artificial light. In contrast, a pyroelectric detector array can be used to identify events with minimal signal processing because it generates a signal only when the temperature changes at some point within the scene, and will do this equally well in daylight or in total darkness.
The present invention provides a passive infrared sensor system comprising: a passive infrared detector comprising a two dimensional array of passive infrared sensing elements and means for focusing infrared radiation from a sense onto the array, such that any event within the scene which results in a change in apparent temperature produces a signal in the element or elements corresponding to the region of the scene in which the event occurs; electronic readout, control, and signal processing circuits connected to the array arranged to identify any elements that produce signals corresponding to an event within the scene and to generate output signals which indicate the occurrence of an event and its location within the scene; and a video camera operating in the visible or near infrared spectral region combined with a directive mechanism operable in response to signals from said readout, control and signal processing circuits to steer the camera in azimuth and elevation towards the location of the event. Thus, in its preferred embodiment the present invention allows the automatic detection capability of passive infrared devices to be combined with a visible camera of high resolution directed to the area or areas of interest.
In the preferred embodiment of the present invention, the image of a scene in which it is required to detect events is focused by a lens or mirror on to a two dimensional array of infra-red devices operating in the whole or a part of the wavelength range 2 xcexcm to 15 xcexcm. The array is formed of pyroelectric sensing elements which detect the changes of temperature due to an event rather than the actual temperature of the elements of the scene. Suitable detector arrays will usually be rectangular and contain between eight and a hundred elements in each dimension, corresponding in a square detector array to between sixty-four and ten thousand elements. The occurrence of a possibly significant event is indicated by a signal from an element of the array. The significance of the event may be further verified by other procedures such as those that are described in UK Patent Application 2340222A.
When an event has been judged to be significant an electronic video camera, operating in the visible or near infra-red, is steered in azimuth and elevation so that its field of view is centred on that portion of the scene which corresponds to the element or elements of the pyroelectric array with an above threshold signal. If a number of significant events are identified by the infrared array the camera may be steered to each event in turn. Alternatively, one of the events may be selected according to pre-determined parameters such as the amplitude of the signals, the apparent size of the event, the order of occurrence of the events, or the relative movements of the events. The camera is standby mode may be focused on the whole scene under surveillance and a zoom lens arrangement may be employed to give better resolution of the element of the scene where the significant event has been identified. The action of the zoom facility may be controlled by a signal generated from the pyroelectric detector indicative of the apparent size of the event within the scene. The camera may also be focused automatically on to the portion of the scene that has been selected. Focusing may be achieved either by a self contained automatic focusing system within the video camera, or by an automatic focusing system which is controlled by a signal generated from the pyroelectric array indicative of the distance of the event from the sensor system.
If the illumination of the scene is inadequate for the video camera, additional illumination may be employed to aid its operation, either globally or directed at the region of scene judged to be significant. The steering mechanism for directed illumination is desirably the same mechanism as is used for the video camera. Where covert operation of the equipment is required the additional illumination may operate in the near infra red, around 0.8 xcexcm wavelength, so that it is not detected by an intruder. Suitable sources of infrared illumination include tungsten filament lamps with appropriate band pass-filters and high power infrared emitting diodes, including diode lasers. The light source may be combined with suitable lenses and/or mirrors to form an infrared or visible searchlight illuminating the desired area.
The electronic video camera may be a CCD camera, or any other electronically scanned array of silicon photodiodes. It may be equipped with storage means, such as a magnetic type, or a silicon DRAM preceded by analogue to digital conversion. Each frame is scanned at a conventional video rate and the frame rate may be a conventional video frame rate, or a lower rate such as 1-5 frames per second to economise on storage. Means may also be provided for recording the event location and/or size. This could, for example be added to the magnetic tape video recording in such a way as to indicate the position and/or size of each recorded event within the scene when the video tape is re-played.
When a signal above a pre-set threshold is generated from one or more elements of the array, the video camera is steered so that its image is centred on the relevant region of the scene. If the camera in standby mode surveys the whole scene that is imaged by the pyroelectric array, a zoom lens arrangement increases the focal length of the camera so that it images only a portion of the scene in the appropriate region. This may be followed by auto focus adjustment. If several separate elements of the array are above threshold the video camera may be steered to each relevant portion of the scene in turn, or it may be directed to one of the events, for example the one giving the largest signal, or the first one to appear. Arrangements may be made to decrease the occurrence of false alarms in the equipment, for example those described in UK patent application number 2340222A, before the video camera is moved.
When the equipment is used to identify intruders, the arrangement ensures the high quality visual images of the intruders and/or their vehicle are made available to operators monitoring the equipment remotely when an alarm signal is generated. Alternatively, these images may be stored, either within the video camera itself or remotely, for subsequent examination. When used to identify a fire, the progress of the fire may be monitored and the appearance of people near the fire may be recorded e.g. arsonists or fire fighters. On the other hand if operators are available the equipment can be used to prevent the alerting of external services when the high quality directed images from the video camera reveal (for example) that the alarm has been triggered by sunlight, automatic central heating equipment, animals or wind blown objects, or by persons with legitimate access to the area. When used to detect the emission of a gas, the discrimination of the pyroelectric array may be enhanced by means of band pass filters to confine it sensitivity to the absorption bands of the gas.
In addition to the video camera, it may be desirable to direct auxiliary equipment towards the location of the event within the scene. One example of auxiliary equipment is a source of illumination, as described above; another example is a water jet, which may be directed at the seat of a fire. Alternatively, the system may include means for displaying or recording the spatial co-ordinates and/or the apparent size of the event, as derived from the signals generated by the pyroelectric sensor system.
In the foregoing, it is assumed that the passive infrared detector is stationary with respect to the scene. However, in another preferred embodiment, the detector itself is movable as well as the video camera. The detector may be steered by the same directive mechanism as the video camera, for example by being mounted with the video camera on a pan and tilt mechanism so that the optical axes of the detector and the camera are coincident. Alternatively, the array may have an independent mechanism. Thus, when an event has been judged to be significant, the array and the camera are steered in azimuth and elevation so that their fields of view are both centred on that portion of the scene in which the event is occurring.
The description, which follows, is for illumination only and does not restrict the scope of the claims.